The present invention relates to an apparatus for machining by electrical discharges an electrode workpiece by means of an electrode tool, in which the relative displacement of the electrodes is effected at least in one direction by means of a servo motor.
The invention is an important improvement on the system disclosed in U.S. Pat. No. 4,049,942, the invention preventing the electrodes from getting any closer to each other than a predetermined distance or gap while maintaining electrical discharges across the gap until the gap becomes too wide to sustain the electrical discharges, and which permits the electrodes to relatively move away from each other as a function of the machining conditions at the gap. As the maximum distance or gap between the electrodes which permits the occurrence of electrical discharges is exactly known, the precise dimension of the machined surface of the workpiece can be determined according to the relative position of the electrodes at the moment the electrical discharges are no longer capable of being sustained due to too wide a gap.
The apparatus of the present invention provides, in addition to controlling the final machined dimension, a control of the relative displacement velocity of the electrodes during a machining operation. The apparatus of the invention comprises means for controlling the speed of the EDM machine servo-motor by means of a first signal which is a function of the machining conditions and by means of a second signal which is a function of the difference, or error, between the relative position of the electrodes and a reference relative position, the second signal being opposed to the first signal at the moment that the reference relative position is reached, such as to limit the relative displacement of the electrodes until a predetermined final relative position is reached. The apparatus of the invention is further characterized in that it comprises means for continuously varying the reference relative position, in the same direction as the direction in which machining is effected, at a predetermined speed, such as to maintain the difference between the reference and actual positions within predetermined limits.
Such an arrangement, as provided by the invention, presents the advantage of accomplishing, by means of a very simple circuit, two additional functions which, until now, were each achieved by a specific and complicated device. Those two functions are, first, storing in a memory the relative position of the electrodes when the electrodes are withdrawn from each other coupled with a slowing down of the relative closure speed of the electrodes when that relative stored position is reached again after resuming normal relative advance of the electrodes and, secondly, automatically limiting the speed of relative feed in order not to exceed a critical intensity of machining current in the course of a machining operation. As soon as the reference position ceases to vary, the control of the invention stops the machining feed according to the method and system disclosed in the above-mentioned U.S. patent. The invention presents particular advantages when it is used to control a plurality of servo-motors which in turn operate simultaneously the relative displacement of the electrodes along several axes. Such is the case, for example, when the relative displacement is effected in a predetermined direction or axis by means of a first servo-motor and in directions or axes perpendicular to that first direction by means of a second servo-motor. The arrangement of the invention permits to synchronize the displacements of the electrodes in each of the axes while permitting to use at its maximum the dynamic characteristics of the servo-motors for withdrawing the electrodes from each other in the event a machining fault is detected.
In addition, by providing a visual display of the instantaneous reference position, a very stable information of the evolution of the machining operation is provided.